1. Field of the Invention
This invention relates to a new method for improving lattice substitution of heteroatoms in large and extra-large pore borosilicate zeolites.
2. Description of the Related Art
Natural and synthetic microporous crystalline molecular sieves including metallosilicates have found widespread industrial applications as catalysts, adsorbents and ion exchangers. These molecular sieves have distinct crystal structures with ordered pore structures which are demonstrated by distinct X-ray diffraction patterns. The crystal structure defines cavities and pores which are characteristic of the different types of zeolite and are similar in size to small organic molecules (generally 3-15 xc3x85). The adsorptive, catalytic and/or ion exchange properties of each molecular sieve depend largely on its large internal surface area and highly distributed active sites, both of which are accessible through uniform molecularly sized channels and cavities.
According to the Structure Commission of the International Zeolite Association, there are over 120 different microporous crystalline molecular sieve structures. The cage or pore size of these materials is denoted by the number of oxygen atoms (likewise the number of tetrahedral atoms) circumscribing the pore or cavity, e.g., a pore circumscribed by n oxygen-atoms is referred to as an n membered-ring pore, or more simply, n-MR. Molecular sieves containing pores and/or cages with molecular-sized windows (containing 8-MR or larger) can have industrial utility in separation, ion exchange and catalysis. Depending on the largest pore openings that they possess, molecular sieves are usually categorized into small (8-MR), medium (10-MR), large (12-MR) and extra-large (xe2x89xa714-MR) pore molecular sieves.
Metallosilicates are molecular sieves with a silicate lattice wherein a metal atom (referred to herein as xe2x80x9cheteroelementxe2x80x9d or xe2x80x9cheteroatomxe2x80x9d) can be substituted into the tetrahedral positions of the silicate framework. Examples of these metals are boron, aluminum, gallium, iron and mixtures thereof. The substitution of boron, aluminum, gallium and iron for silicon results in a change in the balance between the silicon and the corresponding trivalent ions in the framework, thereby resulting in a change of the electrical charge on the framework of the molecular sieve. In turn, such a change in the framework charge alters the ion exchange capacity of a material as well as the adsorptive and catalytic behavior because of the distinct physicochemical properties of these heteroelements. Thus, the utility of a particular molecular sieve, and in particular its adsorptive, catalytic and ion exchange properties, depend largely not only on its crystal structure but also on the properties related to the framework composition. For example, stronger acid strength in zeolite catalysts is required for iso-butane/butene alkylation at lower reaction temperatures to simultaneously achieve higher activity and a lower deactivation rate of the catalyst. By contrast, as demonstrated by S. Namba et al. (Zeolites 11, 1991, p.59) in studies on the alkylation of ethylbenzene with ethanol over a series of metallosilicates with MFI (ZSM-5) zeolite structure, namely, B-ZSM-5, Sb-ZSM-5, Al-ZSM-5, Ga-ZSM-5 and Fe-ZSM-5, the para-selectivity to para-diethylbenzene is largely related to the acid strength of the catalysts and the weaker acid sites provide a higher para-selectivity.
In nature, molecular sieves commonly form as geothermally heated ground water passes through silicate volcanic ash. Early attempts to synthesize zeolites centered around recreating the high-pressure, high-temperature conditions found in nature. Barrer (J. Chem. Soc., 1948, p. 127) demonstrated the first successful zeolite synthesis (mordenite) while Milton (U.S. Pat. No. 2,882,243 (1959)) developed the large-scale zeolite synthesis at low temperatures and pressures that allowed zeolites to gain industrial importance. These zeolite syntheses relied on the presence of alkali metal cations in the synthesis mixture to serve as a mineralizing agent. The alkali metal cations also play a role in the structure direction of the particular zeolite that forms. Building on the concept of cationic structure direction, the range of cations was subsequently expanded later on from the inorganic metal cations to organic cations such as quaternized amines.
Theoretical studies of molecular sieve structures and structure types indicate that only a small fraction of the configurations possible for microporous, crystalline molecular sieves have been discovered. Apparently, the major roadblock in tailoring and utilizing molecular sieve materials for specific applications in catalysis, adsorption and ion exchange is the development of synthesis methods to produce the desirable structure with the desirable framework composition.
In principle, there are two routes leading to the formation of a particular molecular sieve structure with a particular framework composition, e.g., a particular metallosilicate such as aluminosilicate, gallosilicate, ferrosilicate or borosilicate of the same crystal structure: (1) direct synthesis and (2) post-synthetic treatment (secondary synthesis).
Direct synthesis is the primary route for the synthesis of molecular sieves. The major variables that have a predominant influence on the molecular sieve structure include: synthesis mixture composition, temperature and the period of time for which the synthesis is allowed to proceed. Even though each variable contributes to a specific aspect of the nucleation and crystallization during synthesis of a molecular sieve, there is substantial interplay between these elements during the formation of molecular sieves. In the presence of heteroelement X (X=Al, Ga, Fe or B, for example, or X=none for pure-silica molecular sieves), the Si/X ratio will determine the elemental framework composition of the crystalline product; but the amount of the heteroelement in the synthesis mixture also can determine which structure, if any, crystallizes. In addition to the Si/X ratio, various other factors related to the gross composition of the synthesis mixture also play an important role. These factors include: OHxe2x88x92 (or Fxe2x88x92) concentration, cations (both organic and inorganic), the presence of anions other than OHxe2x88x92 (or Fxe2x88x92), and the amount of water in the synthesis mixture. There are also history-dependent factors such as digestion or aging period, stirring, nature (either physical or chemical) of the synthesis mixture, and order of mixing.
In short, depending on the nature of the molecular sieves and the chemistry of their formation, some of these molecular sieve structures can be synthesized using a broad spectrum of framework compositions; such as ZSM-5 containing heteroatoms, i.e., (Si-ZSM-5 or silicalite-1), Al(Al-ZSM-5), B(B-ZSM-5), Fe(Fe-ZSM-5 and Ga(Ga-ZSM-5), whereas the synthesis of other structures succeeds only if certain heteroatoms are present in the synthesis mixture and, in turn, incorporated into the framework. Or, some structures containing specific heteroatom(s) can be synthesized only in a limited range of Si/X ratio. Some structures containing specific heteroatom(s) can be synthesized only if certain specific, usually more expensive, structure-directing agents are employed. These complicated relationships between zeolite structures, framework compositions and structure directing agents have been discussed in many publications and patents, e.g., by Zones et al. in J. Am. Chem. Soc., 122, 2000, p. 263.
U.S. Pat. No. 4,963,337 (xe2x80x9cthe ""337 patentxe2x80x9d) to Zones discloses a procedure for synthesizing borosilicate zeolite SSZ-33 (which is the first synthetic zeolite containing intersected 10- and 12-membered ring channels) by using N,N,N-trimethyl-8-tricyclo[5.2. 1.02,6]decane ammonium cation as a structure-directing agent. Attempts for direct synthesis of aluminosilicate, gallosilicate and ferrosilicate SSZ-33 using this structure-directing agent have not been successful.
U.S. Pat. No. 4,910,006 to Zones et al. discloses a procedure for synthesizing aluminosilicate zeolite SSZ-26 (which has a very similar crystalline structure to SSZ-33) using N,N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x2-hexamethyl[4.3.3.0]propellane-8,11-diammonium cation as a structure-directing agent. However, this structure-directing agent is difficult to make and, hence, much more expensive than N,N,N-trimethyl-8-tricyclo[5.2.1.02,6] decane ammonium cation which is used for the synthesis of borosilicate SSZ-33.
In addition to the preparation of a specific molecular sieve structure with a specific framework composition via the aforesaid direct synthesis, post-synthetic treatments (or secondary synthesis) often provide a more economic alternative route to achieve this goal. The post-synthetic treatment techniques all operate on the same principle: the desirable heteroatoms such as Al, Ga and Fe are inserted into lattice sites previously occupied by other T-atoms such as B. For example, the ""337 patent discloses a method of converting borosilicate SSZ-33 (referred to as B-SSZ-33) into aluminosilicate SSZ-33 (referred to as Al-SSZ-33) with much stronger framework acid sites, by heating a calcined B-SSZ-33 in an aqueous Al(NO3)3 solution at xcx9c100xc2x0 C. As shown in the ""337 patent, Al-SSZ-33 provides a 62% feed conversion for the acid-catalyzed n-hexane/3-methylpentane cracking at 800xc2x0 F. By contrast, due to the low acidity associated with boron-atoms in B-SSZ-33 framework this zeolite has essentially no activity for the same reaction under the same conditions. This illustrates the benefits of making catalytically more active aluminosilicate zeolites from their borosilicate counterparts via post-synthetic treatments.
In summary, to date, direct synthesis is often difficult or impossible for preparing some useful structures of catalytically active alumino-, gallo- or ferrosilicate zeolites. As shown in e.g., the ""337 patent, it is possible to synthesize novel borosilicate zeolite structures. Borosilicate zeolites, however, are not sufficiently catalytically active to be practicable for certain hydrocarbon conversion processes.
Therefore, there remains a need for a method of replacing the boron in borosilicate zeolites with other heteroatoms that may enhance the catalytic activity of the zeolite.
In one embodiment (method A), the invention provides a method for preparing a zeolite having lattice substituted heteroatoms. The method comprises: (a) contacting a calcined large or extra-large pore borosilicate zeolite with an acid, thereby producing an at least partially deboronated zeolite; and (b) contacting the at least partially deboronated zeolite with a salt-containing aqueous solution comprising one or more salts selected from the group consisting of aluminum salt, gallium salt, and iron salt, thereby producing a silicate or borosilicate zeolite having a lattice comprising aluminum atoms; gallium atoms, iron atoms or a combination thereof. Step (b) is conducted at a pH of about 3.5 or less. Preferably, step (a), step (b) or both are conducted at a temperature of from about ambient temperature to about 300xc2x0 C., optionally, under stirring.
In a second embodiment (method C), the invention provides a method of preparing a zeolite having substituted heteroatoms. The method of this second embodiment of the invention comprises contacting a calcined large or extra-large pore borosilicate zeolite with a solution selected from the group consisting of an aqueous aluminum salt solution, thereby producing an aluminosilicate zeolite; an aqueous gallium salt solution, thereby producing a gallosilicate zeolite; an aqueous iron salt solution, thereby producing a ferrosilicate zeolite and mixtures thereof; and wherein said contacting occurs at a pH of not greater than about 3.5. Preferably, the contacting is conducted at a temperature of from about ambient temperature to about 300xc2x0 C., optionally under stirring (method B).
The method of the invention (in all its embodiments, including methods A, B, and C) is particularly efficient for heteroatom lattice substitution in zeolites having a pore size larger than approximately 6.5 xc3x85.
The method of the invention is particularly suitable for producing lattice substituted zeolites comprising an aluminosilicate zeolite selected from the group consisting of SSZ-24, SSZ-31, SSZ-33, SSZ-41, SSZ-42, SSZ-43, SSZ-45, SSZ-47, SSZ-48, SSZ-55, CIT-1, CIT-5, UTD-1, and mixtures thereof, or a gallosilicate zeolite selected from the group consisting of SSZ-24, SSZ-31, SSZ-33, SSZ-41, SSZ-42, SSZ-43, SSZ-45, SSZ-47 SSZ-48, SSZ-55, CIT-1, CIT-5, UTD-1, and mixtures thereof, or a ferrosilicato zeolite selected from the group consisting of SSZ-24, SSZ-31, SSZ-33, SSZ-41, CCS-42, SSZ-43, SSZ-45, SSZ-47, SSZ-48, SSZ-55, CIT-1, CIT-5, UTD-1, and mixtures thereof.